Electric incandescent lamps of the tungsten-halogen variety have proven extremely useful in lighting applications because of their higher efficiency, better maintenance and smaller size in comparison to known incandescent lamps. As a result of this commercial success, tungsten-halogen lamps have been developed with much higher power ratings. These lamps are necessarily larger and present some problems that are not usually encountered with existing incandescent lamps or, for that matter, lower wattage tungsten-halogen lamps. One such problem concerns the adequacy of support for the filament structure within the lamp's light-transmitting envelope. Should proper support not be provided, the filament, typically a coiled tungsten element, can be damaged, rendering the lamp inoperative. Such destruction can result when the lamp is subjected to external forces (i.e., shock) as may occur if the lamp is jarred or dropped (i.e., during shipping). Adequate support is also essential to assure that the filament structure maintains a preestablished, usually centralized, location within the envelope to in turn assure optimum light output for the lamp. This is especially true if the lamp is to be positioned relative to a reflector for directing the lamp's light in a predetermined direction.
In U.S. Pat. No. 3,497,752 (Peterson), assigned to the assignee of this invention, there is shown a filament mount structure wherein the filament, comprised of two coiled elements, is supported only at the ends thereof from a pair of lead-in support rods which in turn are sealed within the lamp's base and project within the envelope's interior. In effect, this seal provides the only support for the support rods and thus the suspended filament.
A subsequent U.S. Pat. No. 3,543,962 (Peterson), also assigned to the same assignee as the instant invention, describes a "folded" filament (one containing several individual coiled elements interconnected at ends thereof by a singular wire) which requires support thereof at the interconnecting (fold) locations. Direct connection of the filament to the support rods, except at the extreme ends thereof, is impossible in a lamp of this type because doing so would electrically short out various parts (segments) thereof.
Tungsten-halogen lamps having such folded coiled filaments, as in U.S. Pat. No. 3,543,962, are sometimes preferable to those having coiled filaments, such as disclosed in U.S. Pat. No. 3,497,752, where it is desired to reduce the effective area of incandescence of the filament as, for example, where the lamp is operated in conjunction with an external reflector and where efficient directional illumination is required, as in motion picture and television lighting. In addition, the corrosive effect of halogen, especially at the high temperatures at which lamps of this type operate, severely limits the materials that can be used within a halogen lamp envelope. Generally, only tungsten and quartz are suitable. In lower wattage lamps, the filament can be welded or hot crimped to the lead-in wire. But, mainly because of the increased wire sizes required, such connections are not usually satisfactory in higher wattage lamps.
The above-mentioned U.S. Pat. No. 3,543,962 discloses a halogen lamp having a bridge which materially stabilizes the central positioning of a folded coiled filament mounted therefrom. The bridge comprises quartz tubing supported by tungsten rods. Filament support wires, inserted through substantially diametral holes in the quartz tubing and supported thereby, insulatively support the filament at the folds thereof. These support wires have sufficient flexibility and adjustability to conveniently permit the takeup of any slack between the quartz tubing and the individual filament folds when the filament is mounted on the bridge. More specifically, one end of each support wire extends through a diametral hole in the quartz tubing and is bent therearound. The opposite end of the support wire extends through and engages a fold of the filament. Between its ends, the support wire has a loop, the loop having sufficient flexibility to permit the legs thereof to be squeezed together. Thus when the filament is mounted on the bridge, any slack in the filament can be taken up by squeezing the loops of the support wires. In some cases, it may be desirable to continue the end of the support wire that engages the filament fold back through the diametral hole in the quartz tubing. Thus, both ends of the support wire extend through the diametral hole and there is effectively a double wire supporting each filament fold. Such a construction can improve the resistance of the filament to sagging, especially when the lamp is operated in a horizontal position.
Preferably, the quartz tubing of each bridge in U.S. Pat. No. 3,543,962 has slots at each end for the purpose of engaging the above-mentioned tungsten rods and to prevent rotation thereof. Such rods, at least in the lamp shown in U.S. Pat. No. 3,543,962, may also serve as the lead-in members for the lamp. Of course, these rods must have sufficient rigidity to permit takeup of filament slack without deflection and to adequately support the filament mounted bridge. Preferably, also, electrical connection is established between the rod and the filament by a compressive connector coil of the type shown in U.S. Pat. No. 3,497,752. Such a coil constrictively encircles the lead-in rod and an abutting portion of the filament leg. Protruding arms of these coils can provide some degree of mount stabilization, but for larger, high power lamps, the coil arms alone are not sufficient for this purpose and have occasionally broken off.
Although the lamp described in U.S. Pat. No. 3,543,962 represented a significant improvement in the art, such a filament mount structure, when employed in large, high-power studio lamps, can exhibit fracture and breakage problems under shock and vibration. Previous approaches for countering this problem included projection of one of the supporting lead-in wires or rods (or a bridge-connected wire) into the exhaust tip to provide rigidity at the free end of the mount. This design was tried without success in large high power lamps, as, during shock and vibration testing, breakage occurred on the bridge member used to anchor the projecting wire. Other prior designs dealt with the mount wobble problem by embedding the support rods at each end of the bulb in a press seal. Such a tungsten-to-quartz seal, however, can lead to cracks at the seal area due to thermal expansion differences, and the approach is somewhat unfeasible for large high-power lamps. Yet another prior method that has been employed to rigidize the mount of smaller, low power lamps involved the use of a wall bumper tungsten wire loop at the free ends of the tungsten support rod, which loop engaged the dome-side area of the lamp envelope. One such example of this engagement is depicted in U.S. Pat. No. 3,898,505 (Danko). This circular loop-support rod construction is deemed not only relatively expensive but, in the case of high power lamps, relatively brittle and thus subject to breakage.
Still a further approach is shown in U.S. Pat. No. 3,626,236 (Robinson et al), wherein internally projecting quartz tubes are sealed to the interior of the envelope, and coils on the upper ends of the support rods fit into these tubes to secure the top of the mount. Although perhaps satisfactory in smaller, lower power lamps with a press seal base, such an upper support means is unsatisfactory for large high power lamp structures. The lever action of the rods or coils on the depending tubes can cause the tubes to break off, especially for the cup seal-supported rod structures typical for high power lamps. Further, the construction appears difficult and expensive to fabricate in production.
In yet another example, as depicted in U.S. Pat. No. 3,909,653 (Bottone et al), the lamp's support rods include V-shaped intermediate sections designed to slidably contact the inner side walls of the envelope. This engagement, also requiring a "spud wire" for being sealed within the envelope's tipped-off section, requires a specifically shaped (oval) envelope to assure centralized filament location. Additionally, such an arrangement is deemed relatively expensive to manufacture.
In U.S. Pat. No. 4,023,060 (Pike et al), also assigned to the same assignee as the present invention, there is described a high power tungsten-halogen lamp wherein the filament, a folded structure, is suspended between bridge members comprised of alumina tubing and supported by a pair of vertically disposed tungsten support rods which in turn are retained in respective cavities provided in the envelope's dome. Such an arrangement, requiring a specially designed envelope and aligned positioning of the parallel rods therein, is also deemed relatively expensive from a manufacturing standpoint.
It is believe, therefore, that an electric lamp, and particularly a tungsten-halogen lamp of the high power variety, posessing an improved means of supporting the filament structure therein which overcomes the several disadvantages associated with the aforementioned lamps would constitute a significant advancement in the art.